1. Field of the Invention
The present invention relates generally to digital-to-analog converter units and, more particularly, to digital-to-analog converter units based on current mirror techniques. In the conventional implementation of a digital-to-analog converter unit, a constant current flows through the unit while the digital input signals steer or divide the current between two conductors, the difference in the divided current resulting in a current mode representation of the digital word or data group.
2. Description of the Related Art
Referring to FIG. 1, the general structure of an digital-to-analog converter unit 10 based on current mirror techniques, according to the related art, is shown. Digital input signals are applied to current mirror 11. Current mirror 11 provides two output currents, I.sub.0 and I.sub.1. The sum of these two output currents is a constant. The digital input signals determine the division of the current between I.sub.0 and I.sub.1. The currents I.sub.0 and I.sub.1 flow through resistors 12 and 13, respectively. The detection unit 14 provides an analog output signal determined the voltages across resistors 12 and 13. This analog output signal is a result of the digital input signals applied to the current mirror 11.
The current mirror-based digital-to-analog converter units rely on the property of a current mirror circuit to replicate a selected unit current. Well designed current mirror units generally have an accuracy of 1%. The current matching between two similar metal-oxide-semiconductor (MOS) transistors operated with similar gate voltages and drain voltages is determined by variations in the diffusion widths, in the poly-silicon widths, in the gate oxide thickness, and ground potential. The present accuracy in current matching was achieved by reducing these variations during the fabrication of the converter units. In spite of improvements, digital-to-analog converter units with even greater accuracy are needed.
A need has therefore been felt for a digital-to-analog converter unit and a related method with a higher accuracy than has been possible with conventional techniques.